This invention relates to a process for preparing polyolefins. The process comprises a first step of premixing a supported boraaryl catalyst with an organoaluminum, followed by olefin polymerization in the presence of the premixed catalyst, an activator, and a second organoaluminum. The organoaluminums may be the same or different. The process is surprisingly useful for the preparation of broad and/or bimodal molecular weight distribution polyolefins.
Interest in metallocene and non-metallocene single-site catalysts has continued to grow rapidly in the polyolefin industry. These catalysts are more active than conventional Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of xcex1-olefin comonomers, and lower polymer density. Examples of non-metallocene single-site catalysts include catalysts containing a boraaryl moiety such as borabenzene, boranaphthalene or boraphenanthrene. See U.S. Pat. No. 5,554,775 and PCT Int. Appl. WO 97/23512.
Unfortunately, the uniformity of molecular weight distribution (MWD) reduces the thermal processing ability of polyolefins made with single-site catalysts. These polyolefins also have a higher tendency to melt fracture, especially at higher molecular weights. These disadvantages combine to make it difficult to process polyolefins produced by single-site catalysts under conditions normally used for Ziegler-Natta polymers. Controllable broadening of MWD is therefore a desired advance in single-site catalyst technology.
One method of increasing processability and broadening MWD of polyolefins produced by single-site catalysts is to physically mix two or more different polyolefins to produce a blended polyolefin mixture with a multimodal, broad molecular weight distribution. For example, see U.S. Pat. No. 4,461,873. In addition, olefin polymerization has been performed in a dual reactor system in order to broaden MWD. The olefin is polymerized by a catalyst in one reactor under one set of conditions, and then the polymer is transferred to a second reactor under a different set of conditions. The first reactor typically produces a high-molecular-weight component, and the second reactor produces a low-molecular-weight component. See U.S. Pat. Nos. 4,338,424, 4,414,369, 4,420,592, and 4,703,094. Lastly, a one-reactor, two-catalyst process has also been used to make multimodal, broad-MWD polymers. The olefin is polymerized in one reactor by two catalysts with different reactivity to form a reactor blend having broad and/or multimodal molecular weight distribution. The catalysts may be either two (or more) separate metallocenes or a metallocene and a Ziegler-Natta component. See, for example, U.S. Pat. Nos. 4,937,299 and 4,530,914, in which at least two separate metallocenes are used in one reactor to form multimodal polymers. See U.S. Pat. Nos. 5,032,562 and 5,539,076 for examples of the metallocene/Zeigler-Natta catalyst mixture in one reactor.
A significant disadvantage of each of these methods is the added cost of using two reactors or two catalysts in the polymerization process. A simpler method would use a single catalyst system that produces broad MWD polymers in a one-reactor process. For example, EP 719,797 A2 discloses an olefin polymerization process in which conventional metallocenes and at least two different co-catalysts are used to produce broad/bimodal MWD polyolefins. In addition, copending application Ser. No. 09/439,462 (U.S. Pat. No. 6,294,626) discloses a method for producing broad and/or bimodal polyolefins using a catalyst comprising an activator and an organometallic compound that incorporates a modified boraaryl ligand.
In sum, new processes are needed. Particularly valuable processes are those that would use one catalyst to produce broad MWD polyolefins having greater thermal processing ability.
The invention is a process for polymerizing olefins. The process comprises preparing a catalyst system by reacting a first organoaluminum with a supported boraaryl catalyst and then polymerizing an olefin in the presence of the premixed catalyst, an activator and a second organoaluminum. The process surprisingly leads to the production of broad MWD polyolefins. The results are particularly surprising since co-pending U.S. application Ser. No. 09/318,009 (U.S. Pat. No. 6,291,386) teaches that olefin polymerization with a boraaryl catalyst produces polyolefins with narrow MWD when organoaluminums are added to the reactor, without a premixing step.
The process of the invention comprises: (a) preparing a catalyst system by premixing a first organoaluminum with a supported catalyst comprising a support and an organometallic compound comprising a Group 3-10 transition or lanthanide metal, M, and at least one boraaryl ligand; and (b) polymerizing an olefin in the presence the catalyst system of step (a), an activator, and a second organoaluminum. The second organoaluminum may be the same as or different from the first organoaluminum.
The supported catalyst of the invention comprises a support and an organometallic compound. The organometallic compound useful in the invention contains at least one boraaryl ligand. Suitable boraaryl ligands include substituted or unsubstituted boraaryl groups, such as substituted or unsubstituted borabenzenes, boranaphthalenes or boraphenanthrenes, as described in U.S. Pat. No. 5,554,775, the teaching of which is incorporated herein by reference. The metal, M, may be any Group 3 to 10 transition or lanthanide metal. Preferably, the catalyst contains a Group 4 to 6 transition metal; more preferably, the catalyst contains a Group 4 metal such as titanium or zirconium.
The transition or lanthanide metal may also have other polymerization-stable anionic ligands. Suitable ligands include cyclopentadienyl (substituted or unsubstituted) anions such as those described in U.S. Pat. Nos. 4,791,180 and 4,752,597, the teachings of which are incorporated herein by reference. Suitable ligands also include another boraaryl group or a substituted or unsubstituted azaborolinyl, pyrrolyl, indolyl, quinolinyl, hydroxypyridinyl, or aminopyridinyl group as described in U.S. Pat. Nos. 5,554,775, 5,539,124, 5,637,660, and 5,902,866, the teachings of which are also incorporated herein by reference.
The boraaryl ligand and the other polymerization-stable anionic ligand can be bridged. Groups that can be used to bridge the ligands include, for example, methylene, ethylene, 1,2-phenylene, dialkylsilyls, and diarylsilyls. Normally, only a single bridge is used in the single-site catalyst, but complexes with two bridging groups can be used. Bridging the ligand changes the geometry around the transition metal and can improve catalyst activity and other properties, such as molecular weight, comonomer incorporation, and thermal stability.
Other suitable ligands include halides and C1-C20 alkoxy, siloxy, or dialkylamido ligands. Particularly preferred ligands are halides.
The organometallic compound is immobilized on a support to form the supported catalyst of the invention. The support is preferably a porous material. The support can be inorganic oxides, inorganic chlorides, and organic polymer resins, or mixtures thereof. Preferred inorganic oxides include oxides of Group 2, 3, 4, 5, 13, or 14 elements. Preferred inorganic chlorides include chlorides of the Group 2 elements. Preferred organic polymer resins include polystyrene, styrene-divinylbenzene copolymers, and polybenzimidizole. Particularly preferred supports include silica, alumina, silica-aluminas, magnesias, titania, zirconia, magnesium chloride, and polystyrene.
Preferably, the support has a surface area in the range of about 10 to about 700 m2/g, more preferably from about 50 to about 500 m2/g, and most preferably from about 100 to about 400 m2/g. Preferably, the pore. volume of the support is in the range of about 0.1 to about 4.0 mL/g, more preferably from about 0.5 to about 3.5 mL/g, and most preferably from about 0.8 to about 3.0 mL/g. Preferably, the average particle size of the support is in the range of about 10 to about 500 xcexcm, more preferably from about 20 to about 200 xcexcm, and most preferably from about 10 to about 100 xcexcm. The average pore diameter is typically in the range of about 10 to about 1000 xc3x85, preferably about 20 to about 500 xc3x85, and most preferably about 50 to about 350 xc3x85.
The organometallic compound is supported using any of a variety of immobilization techniques. In one method, organometallic compound is dissolved in a solvent and combined with the support. Evaporation of the solvent gives a supported catalyst. An incipient wetness method can also be used.
The support can be used without any pre-treatment prior to immobilization of the organometallic compound, but a support pre-treatment step is preferred. The support may be calcined and/or modified by a chemical additive. If the support is pre-treated by calcination, the calcination temperature is preferably greater than 150xc2x0 C. The chemical additives used to pre-treat the support include triaklylaluminums, alumoxanes, organoboranes, organomagnesiums, organosilanes, and organozinc compounds. Support modification techniques are taught in U.S. Pat. Nos. 4,508,843, 4,530,913, and 4,565,795, the teachings of which are incorporated herein by reference.
Preferably, the support is silylated prior to use. Silylation is used to remove acidic sites from the support surface. Silylation is performed by reacting the support with a silylating agent, either in solution by incipient wetness or impregnation, or in the vapor phase. Preferred silylating agents include alkylsilyl halides, alkyldisilazanes, alkyl and aryl alkoxysilanes. Preferred alkylsilyl halides include trialkylsilyl halides, dialkylsilyl dihalides, and alkylsilyl trihalides, which preferably have the formula R3R4R5SiX, R3R4SiX2 or R3SiX3. Particularly preferred alkylsilyl halides are trimethylchlorosilane, dimethyldichlorosilane, t-butyldimethylchlorosilane, and trimethylsilyl iodide.
Suitable alkyl disilazanes include hexaalkyl disilazanes having the formula R33SiNHSiR33. In particular, hexamethyldisilazane is preferred.
Preferred alkyl or aryl alkoxysilanes include trialkyl alkoxysilanes, dialkyl dialkoxysilanes, and alkyl trialkoxysilanes, which preferably have the formula R3R4R5Si(OR6), R3R4Si(OR5)(OR6) or R3Si(OR4)(OR5)(OR6) where R3, R4, R5, and R6 denote the same or different C1-C20 hydrocarbyl. Exemplary alkyl alkoxysilanes are cyclohexylmethyldimethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, phenyltriethoxysilane, and dicyclopentyldimethoxysilane.
Optionally, the support can be treated with an organoboron compound following silylation. Preferred organoboron compounds include trialkylborons, triarylborons, and trialkoxyborons having the formula R3R4R5B or B(OR3)(OR4)(OR5). Most preferred are trimethylboron, triethylboron, tripropylboron, triisobutylboron, trimethoxyboron, triethoxyboron, tripropoxyboron, and triphenoxyboron.
The organoboron compound is added to the support in an amount preferably in the range of about 0.1 to 10 mmoles of boron per gram of support, more preferably from about 0.2 to 5 mmoles/gram, and most preferably from about 0.5 to 3 mmoles/gram. Treatment with the organoboron compound may be performed in either the liquid phase or in the vapor phase. In the liquid phase, the organoboron compound is applied to the support as a liquid, either by itself or as a solution in a suitable solvent such as a hydrocarbon. In the vapor phase, the organoboron compound is contacted with the support in the form of a gas or by injecting liquid modifier into the preheated support to vaporize the modifier. Treatment temperatures are preferably in the range of from about 20xc2x0 C. to about 400xc2x0 C. The organoboron treatment step can be carried out in a batch, semi-continuous, or continuous manner.
The support is preferably heated at a temperature from about 50xc2x0 C. to about 1000xc2x0 C., more preferably from about 100xc2x0 C. to about 800xc2x0 C., either before or after the organoboron modification. In another method, the support heat treatment and organoboron modification occur simultaneously as the organoboron compound in the vapor phase is passed over a heated support as discussed above.
Before addition into the olefin polymerization reactor, the supported catalyst is premixed with a first organoaluminum to form the catalyst system of the invention. The first organoaluminum is a trialkyl or triaryl aluminum compound, which preferably has the formula AIR13 where R1 denotes a C1-C20 hydrocarbyl. Most preferably, the first organoaluminum is trimethyl aluminum (TMA), triethyl aluminum (TEAL), or triisobutyl aluminum (TiBAL). The premixing can be accomplished by a variety of methods. For example, the supported catalyst can be mixed in a solution containing the organoaluminum. Alternatively, the organoaluminum can be added to the support by an incipient wetness technique.
The premixed catalyst system is injected into a reactor containing an olefin monomer, an activator and a second organoaluminum. The second organoaluminum is also a trialkyl or triaryl aluminum compound, which preferably has the formula AIR13 where R1 denotes a C1-C20 hydrocarbyl, and most preferably is trimethyl aluminum, triethyl aluminum, or triisobutyl aluminum. The second oganoaluminum may be the same as or different from the first organoaluminum.
The process of the invention is also performed in the presence of an activator. Suitable activators include ionic borates and aluminates such as tri(n-butyl)ammonium tetrakis(pentafluorophenyl)borate, trityl tetrakis(pentafluorophenyl)borate or trityl tetrakis(pentafluorophenyl)aluminate. The molar ratio of the metal of the activator component to the transition metal of the single-site catalyst is preferably in the range of about 0.1:1 to 10:1, and more preferably from about 0.3:1 to 3:1.
The process of the invention is used to polymerize olefins, preferably xcex1-olefins. Suitable olefins include, for example, ethylene, propylene, 1-butene, 1-hexene, 1-octene, and the like, and mixtures thereof. The process is valuable for copolymerizing ethylene with xcex1-olefins or di-olefins (e.g., 1,3-butadiene, 1,4-hexadiene, 1,5-hexadiene).
The process of the invention is preferably a liquid phase (slurry, solution, suspension, bulk) process. The pressure in the polymerization reaction zones typically ranges from about 15 psia to about 15,000 psia, and the temperature usually ranges from about xe2x88x92100xc2x0 C. to about 300xc2x0 C. Slurry phase processes are preferred. A slurry process involves pressures in the range of about 1 to about 500 atmospheres and temperatures in the range of about xe2x88x9260xc2x0 C. to about 100xc2x0 C. The reaction medium employed should be liquid under the conditions of polymerization and relatively inert. Preferably, it is an alkane, a cycloalkane, or an aromatic hydrocarbon such as toluene, ethylbenzene, or xylene. More preferably, hexane or isobutane is employed.